Experimental and ab initio infrared study of χ-, κ- and α-aluminas formed from gibbsite

Article abstract of DOI:10.1016/j.jssc.2010.02.010

χ-, κ- and α-alumina phases formed by dehydration of micro-grained gibbsite between 773 and 1573 K are studied using infrared spectroscopy (IR). The structural transitions evidenced by X-ray diffraction (XRD) were interpreted by comparing IR measurements with ab initio simulations (except for the χ form whose complexity does not allow a reliable simulation). For each phase, IR spectrum presents specific bands corresponding to transverse optical (TO) modes of Al-O stretching and bending under 900 cm^-1. The very complex χ phase, obtained at 773 K, provides a distinctive XRD pattern in contrast with the IR absorbance appearing as a broad structure extending between 200 and 900 cm^-1 resembling the equivalent spectra for γ-alumina phase. κ-alumina is forming at 1173 K and its rich IR spectrum is in good qualitative agreement with ab initio simulations. This complexity reflects the large number of atoms in the κ-alumina unit cell and the wide range of internuclear distances as well as the various coordinances of both Al and O atoms. Ab initio simulations suggest that this form of transition alumina demonstrates a strong departure from the simple pattern observed for other transition alumina. At 1573 K, the stable α-Αl₂?₃ develops. Its IR spectra extends in a narrower energy range as compared to transition alumina and presents characteristics features similar to model α-Αl₂?₃{dot operator} Ab initio calculations show again a very good general agreement with the observed IR spectra for this phase. In addition, for both κ- and α-Αl₂?₃, extra modes, measured at high energy (above 790 cm^-1 for κ and above 650 cm^-1 for α), can originate from either remnant χ-alumina or from surface modes.

Full text of DOI:10.1016/j.jssc.2010.02.010

ARTICLE IN PRESS
Journal of Solid State Chemistry 183 (2010) 901–908
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Experimental and ab initio infrared study of
from gibbsite
w
-,
k
- and -aluminas formed
a
a
a,b
c
d
a
c
a
a,
Ã
L. Favaro , A. Boumaza , P. Roy , J. L e´ dion , G. Sattonnay , J.B. Brubach , A.M. Huntz , R. T e´ tot
a
ICMMO(LEMHE), UMR 8182 CNRS-Universit e´ Paris-Sud 11-Orsay, F-91405, France
b
Laboratoire des Structures, Propri e´ t ´e s et Interactions Inter Atomiques (LASPI2A), Centre Universitaire de Khenchela 40000, Algeria
c
Synchrotron SOLEIL, L’Orme des Merisiers, BP 48-Gif-sur-Yvette CEDEX, F-91192, France
d
LIM, UMR CNRS, ENSAM, 151 Bd de l’H oˆ pital, Paris, F-75013, France
a r t i c l e i n f o
a b s t r a c t
Article history:
w k- and a-alumina phases formed by dehydration of micro-grained gibbsite between 773 and 1573 K
-,
are studied using infrared spectroscopy (IR). The structural transitions evidenced by X-ray diffraction
XRD) were interpreted by comparing IR measurements with ab initio simulations (except for the
Received 26 January 2010
Accepted 11 February 2010
Available online 17 February 2010
(
w
form whose complexity does not allow a reliable simulation). For each phase, IR spectrum presents
Keywords:
specific bands corresponding to transverse optical (TO) modes of Al–O stretching and bending under
À1
Gibbsite
900 cm . The very complex
w
phase, obtained at 773 K, provides a distinctive XRD pattern in contrast
Transition alumina
À1
with the IR absorbance appearing as a broad structure extending between 200 and 900 cm
resembling the equivalent spectra for -alumina phase.
spectrum is in good qualitative agreement with ab initio simulations. This complexity reflects the large
number of atoms in the -alumina unit cell and the wide range of internuclear distances as well as the
w-alumina
k-alumina
a-alumina
g
k-alumina is forming at 1173 K and its rich IR
k
Infrared spectroscopy
various coordinances of both Al and O atoms. Ab initio simulations suggest that this form of transition
X-ray diffraction
alumina demonstrates a strong departure from the simple pattern observed for other transition
Ab initio simulations
alumina. At 1573 K, the stable
a
-Al
2
O
3
develops. Its IR spectra extends in a narrower energy range as
-Al U Ab initio
compared to transition alumina and presents characteristics features similar to model
a
2 3
O
calculations show again a very good general agreement with the observed IR spectra for this phase. In
À1
addition, for both
k
- and
a
-Al
2
O
3
, extra modes, measured at high energy (above 790 cm
for k and
À1
above 650 cm
for
a), can originate from either remnant
w
-alumina or from surface modes.
&
2010 Elsevier Inc. All rights reserved.
1
. Introduction
Applications of alumina are widespread in domains such as
tion of the phases present at a given time, in particular by X-ray
diffraction (XRD), which provides similar patterns for various
alumina forms [2]. In a previous paper [3], we studied
g-, d- and
protective barriers, catalytic supports, electronic-device fabrication,
cutting-tools and surgical materials. This diversity in the applica-
y-transition phases prepared from heat treatment of boehmite, and
we showed that combining use of both XRD and infrared spectro-
scopy (IR) allows characterizing transition alumina formed at
different temperatures. In the present paper, the alumina phases
tions is due to the fact that Al
variety of metastable structures, the so-called transition alumina
such as ), as well as the stable -Al phase [1], each
2 3
O has the particularity to exist in a
(
w,
k,
g
,
d
,
y
,
Z
a
O
2 3
(w-, k- and a-alumina) synthesized by heating micro grain gibbsite
having specific physico-chemical and mechanical properties. During
high temperature oxidation of aluminum or alumina-forming alloys,
powder have been studied by these two techniques and analysed
using ab initio calculations. Reaction pathways for the dehydration of
gibbsite have been extensively investigated [4–8]. These studies
showed that the product depends on the atmosphere and grain size
of the starting gibbsite. It is generally accepted that, in air, fine-
2 3
before reaching the stable a-Al O phase, one or several transition
alumina may be formed, depending on the oxidizing atmosphere
and the temperature cycle. The first stages of oxidation and the role
played by the intermediate steps on the properties of the alumina
film is an important task in materials science and is not clearly
understood. One of the difficulties rests in the precise characteriza-
grained gibbsite is transformed to
coarse-grained gibbsite, especially in the presence of water vapor, is
transformed to boehmite and then to -, -, - or -alumina,
w-, k- or a-alumina, whereas
g
d
y
a
depending on the temperature, as shown in Fig. 1. Following this
phase diagram, we prepared four samples at 773, 1173, 1273 and
Ã
1573 K from micro-grained gibbsite, expecting synthesize
a-alumina respectively.
w-, k- and
Corresponding author.
E-mail address: robert.tetot@u-psud.fr (R. T e´ tot).
0022-4596/$ - see front matter & 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2010.02.010

ARTICLE IN PRESS
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L. Favaro et al. / Journal of Solid State Chemistry 183 (2010) 901–908
*
*
*
*
*
α
3 parameters functional for exchange energy [11]. The 85–11G(*)
basis set for aluminum [12] and the 88–31G(*) basis set for oxygen
χ
Κ
Gibbsite
[13] were used. The shrinking factors were set to 4 4 4, correspond-
Boehmite
γ
δ
θ
α
ing to 30 K-points in the irreducible Brillouin zone. The integration
grid was set by the keyword ‘‘XLGRID’’, defining a pruned grid
composed of 75 radial points and 974 angular points. The vibrational
spectra were computed after a full structural optimization as
described by Pascale et al. [14]. The convergence criteria for
optimization steps were, on one side, 0.00003 for the maximum
3
00
500
700
900
T(K)
1100
1300
1500
Fig. 1. Sequence of dehydration and transformation of alumina from gibbsite and
component of displacement and 0.00012 for the maximum mean
boehmite [2,5,7,8]. Stars indicate working temperatures.
À10
root square of the forces and, on another side, 10
Hartree as SCF
energy convergence criterion. These tight convergence choices
ensured the vibrational spectrum to be calculated with a precision
of approximately 10 cm . The precision on intensities should be
within a factor of 2. For the study of transmission through samples,
only the transverse optical modes are compared with measure-
ments. Extra details on the calculation of vibrational frequencies of
crystalline compounds with Crystal code can be found in Refs.
The paper is structured as follows. In Section 2, experimental
(
heat treatment, XRD and IR techniques) and ab initio techniques
are detailed. In Section 3, results obtained on gibbsite, -, - and
-alumina are presented. We conclude in Section 4.
À1
w
k
a
2
. Experimental and modelling techniques
[9,14,15].
2.1. Experimental
3
. Results
Gibbsite
bo (98% purity) was used to form metastable
and stable -alumina through calcinations at various tempera-
g
-Al(OH)
3
micrometric Rectapur powder from Prola-
w
- and -aluminas
k
3.1. Gibbsite
a
tures (see Fig. 1). The gibbsite powder was calcined in air at
temperatures ranging from 773 to 1573 K according to the
following heat treatment:
Gibbsite (
b=5.078, c=9.736 A,
the unit cell contains eight Al(OH)
g
-Al(OH)
3
)
has monoclinic symmetry (a=8.684,
=94,541) with the space group P2 /n, and
units [16]. Gibbsite is
˚
b
1
3
ꢀ
ꢀ
ꢀ
heating up to the specified temperature at 5 K/min,
maintaining for 24 h at the calcination temperature,
cooling down rapidly to room temperature (air quench).
characterized by the stacking of two-layer units (AA or BB) of
hydroxyl sheets with the sequence ABBAABBAy where hydroxyl
sheets of the adjacent Gibbsite layers face the c direction [17]. In
Fig. 2a, the XRD pattern obtained on the as received gibbsite
powder shows a good agreement with the reference XRD pattern
For temperatures lower than 1323 K, the uncertainty on
(
33-0018 JCPDS file). The IR spectrum obtained for the same
temperature was
DT= 72 K, while for temperatures above
sample is presented Fig. 2b. In the high energy range, four strong
1
373 K, the uncertainty on the temperature was larger,
DT= 710
À1
absorption bands (3397, 3467, 3529 and 3623 cm ) are in very
K. All the samples were then characterised using both XRD and IR
spectroscopy. For IR experiments, 0.01–0.10 mg of the ground
powder was mixed with 2372 mg of CsBr and compressed in a
cold 150 MPa isostatic press (CIP) in order to obtain a 200–250
thick pellet.
good agreement with the experimental and theoretical spectra by
Balan et al. [18] and Kloprogge et al. [5, 6] in the range of the
OH-stretching bands of gibbsite. At lower energy, bands
mm
À1
extending in the 250–1000 cm
range may be attributed first
to the bending and then to the stretching of Al–O bonds, while the
À1
strong band at 1020 cm
deformation [5].
probably results from hydroxyl
ꢀ
ꢀ
XRD was performed with a PANalytical X’Pert Pro MRD
diffractometer with CuK radiation ( =0.15406 nm). Data
were collected with steps of 0.021 (2 ).
a
l
y
IR spectra were obtained using a Perkin-Elmer spectrometer at
3.2. Phase transitions induced by heat treatment of gibbsite
À1
8
cm resolution. Fourier transform infrared (FTIR) technique
À1
was used in the transmission mode in the 200–4000 cm
range. For each spectrum, 120 scans were co-added. The
transmission spectra were obtained by dividing the signal
transmitted through the pellet (I) by the signal measured
When heating up fine-grained gibbsite, most OH groups are
eliminated, and various forms of alumina are formed with the
sequence: gibbsite - -- -- -Al when temperature in-
w
k
a
2 3
O
creases. In order to study these phases, we performed XRD and IR
measurements on four samples prepared from gibbsite calcined
for 24 h at 773, 1173, 1273 and 1573 K respectively.
through an aperture of the same dimensions (I
are presented in absorbance (A= Àln(I/I )) as a function of the
incident wavenumbers . The apparatus was purged by dry
nitrogen to minimize absorption by water vapor and CO gas.
0
). All IR spectra
0
o
According to Fig. 1, at 773 K, the
many investigations since the 1950’s [4, 17, 19–21], the crystal
structure of -alumina is still uncertain. Stumpth et al. [19] assumed
w phase is expected. In spite of
2
w
˚
a cubic (not spinel) unit cell of lattice parameter a=7.95A (04-0880
JCPDS file). On the other hand, two hexagonal structures have been
2.2. Ab initio modelling
suggested, either with the parameters a=5.56 A˚ and c=13.44 A˚ [21]
˚
˚
or with a=5.57A and c=8.64A [20] (13-0373 JCPDS file). The two
previous hexagonal unit cells may be described respectively as a
stacking of 6 and 4 close-packed oxygen layers, of approximately the
The equilibrium structural parameters and
G-point vibrational
2 3
frequencies of a- and k-Al O bulk have been calculated using the
˚
periodic ab initio program Crystal 06 [9], which adopts an
all-electron Gaussian-type basis set. The calculations have been
performed at the DFT level using the B3LYP functional, based on
the Vosko–Wilk–Nusair correlation potential [10] and the Becke’s
same thickness (2.24 and 2.16 A) as the Al–OH layers in gibbsite
˚
(2.12 A). More recently, Kogure [17] proposed a hexagonal lattice
˚
with a=4.9A and an undefined c length indicating that
w-alumina
structure can be regarded as random close packing of gibbsite-like

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903
10
20
30
40
50
60
70
80
1
1
50
00
Experimental
γ
5
0
χ
2
2
5
0
0
4
2
0
χ (cubic)
χ (hex.)
100
10
20
30
40
50
60
50
2
θ (deg.)
1
1
0
0
0
0
.2
.0
.8
.6
.4
.2
0
1
00
γ
5
0
0
1
0
20
30
40
50
60
70
80
2
θ (deg.)
Fig. 3. XRD patterns: (a) alumina formed from gibbsite calcined for 24 h at 773 K
expected phase) and -alumina formed from boehmite [18], (b) -aluminas
from cubic JCPDS 04-0880 file and hexagonal JCPDS 13-0373 file (the assignment
(
w
g
w
0
500 1000 1500 2000 2500 3000 3500 4000
refers to the hexagonal structure), (c) g-alumina from JCPDS 10-0425 file.
-1
Wavenumber (cm )
Fig. 2. Gibbsite: (a) XRD measured and from 33-0018 JCPDS file, (b) IR absorbance
spectrum.
2.0
773K
layers. The XRD pattern of 773 K sample is shown Fig. 3a together
with the
patterns (04-0880 JCPDS and 13-0373 JCPDS files) corresponding to
the cubic and hexagonal structures respectively (Fig. 3b).
g
pattern from [3] and is compared with the two reference
1.5
w
Moreover, according to Fig. 1, in particular experimental
conditions, heating gibbsite may also result in the formation of
1.0
g
phase via the transition gibbsite-boehmite [4, 7]. To check this
pathway, the reference XRD pattern (10-0425 JCPDS file) of
-alumina is reported Fig. 3c. The comparison of experimental
g
g
0.5
and
at 2
w
y
-Al
=431, present in
reference pattern (Fig. 3c), allows an unambiguous assignment
to the phase for the 773 K sample. Fig. 3a also shows that sample
is poorly crystallized as suggested by the width of the peaks, most
likely due to the intrinsic disordered character of the phase. As a
2
O
3
XRD patterns (Fig. 3a), and in particular the band (202)
w
reference pattern (Fig. 3b) and not observed
in
g
0.0
w
w
0
200
400
600
800
1000
1200
-1
Wavenumber (cm )
w
consequence, it is somewhat risky to choose between the cubic and
Fig. 4. IR absorbance spectrum of alumina formed from gibbsite calcined for 24 h
the hexagonal form, in spite of the presence of a small peak at
at 773 K.
2y=401 which can suggest the cubic phase. IR absorption has been
measured on the same 773 K sample and is presented in Fig. 4. It
À1
appears as a wide unresolved pattern extending from 350 to
750 and 830 cm . It should be pointed out that the present
À1
8
8
50 cm
with maximum absorbance around 410, 600 and
w
-alumina contains small amounts of hydroxyl ions as
demonstrated by the presence of a band around 1080 cm
characteristics of a bending mode [5]. The similarities with the
À1
À1
00 cm . This spectrum resembles closely the
g
-Al O
2 3
phase [3]
and, as such, seems characteristic of a complex and disordered
crystallographic structure. However, in this case, the broad band
g
phase absorbance spectrum suggests that the lowest energy broad
À1
À1
around 800 cm
contains two substructures centered on around
bands (around 400 cm ) can be assigned to Al–O bending modes,

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L. Favaro et al. / Journal of Solid State Chemistry 183 (2010) 901–908
À1
while the two higher energy structures (600 and 800 cm ) are
1273 K samples (Figs. 6a) show a large number of well resolved
bands in a wide energy range extending from 200 to 900 cm . In
order to analyse these spectra, we have calculated the infrared
transverse optical modes using ab initio methods described
before. These bands are represented Fig. 6b. as sticks whose
length is proportional to their oscillator strength. To help the
À1
dominated by stretching Al–O modes for the AlO
AlO tetrahedron respectively [3]. The band positions are presented
in Table 1 (first column).
For samples prepared at 1173, and 1273 K, the
expected (see Fig. 1). Contrary to the phase, the crystal structure
of -alumina is well known (see for example Ref. [22] and
references therein). -alumina is orthorhombic with the space
group pna2 and results in ten independent atoms positions (four
Al and six O). In Table 2 are displayed the main characteristic of
the phase: atomic parameters, distances of Al–O bonds and
coordinances of atoms. Among the four Al, three are six-
coordinated (AlO octahedra) and one is four-coordinated (AlO
tetrahedra). Among the three AlO octahedra, two of them present
one particularly long Al–O bond (2.27 A for Al
with respect to the five other bonds. In the following, these two
AlO units will be noted as AlO (5+1) (see Table 2). Concerning
6
octahedron and
4
k
phase is
w
k
comparison with experiment it is also represented as a sum of
À1
k
20 cm
width Gaussians.
1
As a preliminary step we have separated the calculated
absorption range in six regions, according to their dominant
contributions reported in Table 1, second column. Accordingly, in
the discussion, we report for each energy region, the calculation
main findings and use them as a guide for attributing the
equivalent region in the experimental spectra.
k
6
4
6
˚
˚
4
, 2.15 A for Al )
2
À1
I. The lowest energy region (
or200 cm ) is dominated by
6
6
contributions from Al–O stretching modes involving five-
coordinated oxygen (OV) and Al (5+1), the two main ones
calculated at 147 and 180 cm . This unusual low energy for a
oxygen atoms, two are four-coordinated, three are three-
coordinated, and one is five-coordinated, which is specific of
this phase. The experimental XRD patterns measured on the 1173
and 1273 K samples and compared with the calculated reference
À1
Al–O stretching mode may be the signature of the loosely
bound Al–O with very large interatomic distances (2.15 and
2.27 A˚ , see Table 2) determined for OV. This region is not
pattern of
k
-alumina are shown in Fig. 5a–c. The experimental
XRD pattern at 1173 K is specific of a pure
contrast with the 1273 K sample in which some minor bands,
noted in Fig. 5b, indicate the presence of a small quantity of
k
-alumina (Fig. 5a), in
covered by our experimental measurements limited to
À1
o
Z200 cm
and is not reported in Table 1.
À1
II. The second energy region (from 200 to 440 cm ) is dominated
by Al–O bending as for other forms of alumina ([3] and
references therein). However in this case, the Al–O pairs largely
belong to AlO octahedra, the Al–O bending for AlO tetrahedra
a
-alumina. Nevertheless, the presence of remnant
be excluded as all the peaks also appear in structure. Note that
the experimental XRD patterns show well crystallized phases, in
contrast with the phase. As a consequence of the great variety of
w-phase cannot
w
k
6
4
w
contributing only to small bands. Nevertheless, the two smaller
À1
crystallographic sites, the infrared spectrum is expected to be
complex. Indeed, the infrared absorbance spectra for 1173 and
contributions calculated at 209 and 236 cm , attributed to
Al–O bending mode for AlO6 octahedra and AlO4 tetrahedra
Table 1
À1
2 3 2 3 2 3
Experimental (Exp) band positions (cm ) for alumina formed from gibbsite calcined for 24 h at 773 K (w-Al O ), 1273 K (k-Al O ) and 1573 K (a-Al O ), together with
assignments from ab initio calculations for
k-and
a
2
-Al O
3
(BꢁAl–O bending, SꢁAl–O stretching).
Chi
Kappa
Alpha
Exp.
Exp.
Assignment (ab initio calc.)
Exp.
Assignment (ab initio calc.)
240
270
300
335
372
407
Region II
(200–440)
AlO
+
6
(B)
(B)
386
AlO
4
6
AlO (B)
ꢂ410
422
455
449
4
5
5
5
87
06
36
63
Region III
484
Surface mode?
(440–630)
AlO
+
6
(S+B)
(S+B)
ꢂ590
ꢂ610
597
630
594
640
AlO
4
6
AlO (S)
6
6
41
57
6
7
90
10
Region IV
(630–750)
ꢂ750
737
AlO
6
(S)
782
Region V
(
630–750)
650–800
Surface modes or water
AlO (S)
4
819
Region VI
ꢂ830
838
(790–930)
Mixed contribution
Surface modes ?
Al–O–H groups (B)
ꢂ880
ꢂ1080
1070
1070
Al–O–H groups (B)
Energy regions II, III, IV, V and VI are defined in Fig. 6.

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L. Favaro et al. / Journal of Solid State Chemistry 183 (2010) 901–908
905
Table 2
10
20
30
40
50
60
70
80
2 3
Atomic parameters and interatomic distances in k-Al O .
Exp. 1173K
Atomic parameters for
k
-Al
2
O
3
1
(Space group: Pna2 )
4
2
00
00
0
a=4.8437 A˚ b=8.3300 A˚ c=8.9547 A˚
a/b=0.5815 b/c=0.9302 c/a=1.8487
V=361.30 A˚ 3
Atom
x/a
y/b
z/c
Al1
Al2
Al3
Al4
O1
O2
O3
O4
O5
O6
0.679
0.185
0.812
0.668
0.329
0.025
0.472
0.515
0.861
0.336
0.842
0.343
0.649
0.470
0.831
0.491
0.665
0.673
0.330
0.499
0.000
0.787
0.697
0.999
0.893
0.629
0.638
0.121
0.866
0.900
300
Exp. 1273K
2
1
00
00
α
α
Distances
Atom1
Atom2
˚
Unit
d
12(A)
0
Al1
Al2
O6
O1
O4
O1
O4
O2
1.770
1.877
1.947
1.950
1.959
1.963
AlO
6
Calc.
1
00
50
0
O5
O6
O5
O2
O4
O4
1.725
1.802
1.818
2.030
2.083
2.270
AlO
6
(5+1)
1
0
20
30
40
50
60
70
80
2
θ (deg.)
Al3
Al4
O3
O1
O2
O3
1.735
1.765
1.780
1.811
AlO
AlO
4
Fig. 5. XRD patterns: (a) alumina formed from gibbsite calcined for 24 h at 1173 K
(
expected
k
phase), (b) alumina formed from gibbsite calcined for 24 h at 1273 K,
-alumina described in Table 2.
(
c) reference calculated pattern of
k
O3
O6
O5
O2
O1
O4
1.809
1.852
1.911
1.916
2.063
2.145
6
(5+1)
respectively, may correspond to the two small experimental
(see Table 1). It seems that this
energy region may be compared with the experimental energy
range between 200 and 460 cm
Fig. 6a). Indeed, as predicted by calculations, the oscillator
À1
peaks at 240 and 270 cm
À1
(first measured area in
O1
O2
Al3
Al1
Al1
Al4
1.765
1.877
1.950
2.063
OIV
OIV
strength raises as the energy increases.
À1
III. The next theoretical energy region (from 440 to 630 cm ) is
characterized by the highest intensity bands both experimen-
tally and theoretically with all calculated bands resulting from
mixed contributions from Al–O bending and stretching
Al3
Al4
Al1
Al2
1.780
1.916
1.963
2.030
4 6
motions for both AlO and AlO . The four main calculated
structures as well as their attributions are as follows:
À1
ꢀ
the band at 483 cm
is a mixed contribution of Al–O
(5+1) and AlO . Although this
O3
O4
Al3
Al4
Al3
1.735
1.809
1.811
OIII
OV
stretching between AlO
6
4
assignment is coherent with the previous observation of
Al–O stretching for AlO for the other alumina, the energy
6
Al1
Al1
Al2
Al4
Al2
1.946
1.959
2.083
2.145
2.270
4
is somewhat low for Al–O stretching mode for AlO [3].
À1
ꢀ
the band at 564 cm
results from two unresolved
À1
structures at 562 and 565 cm . The first one can be
assigned to Al–O bending for AlO (5+1), while the second
is a pure Al–O stretching mode for AlO . Again this energy
6
O5
O6
Al2
Al2
Al4
1.725
1.818
1.911
OIII
OIII
4
seems low for this assignment by analogy with other
alumina phases.
the band at 593 cm
À1
ꢀ
ꢀ
is a mixed contribution of Al–O
Al1
Al2
Al4
1.770
1.802
1.852
6
stretching and bending for AlO (5+1).
À1
the intense band at 621cm involves stretching and bending
of all coordinance Al and O atoms.
À1
Experimentally, the energy range between 480 and 670 cm
,
dominated by seven bands (see Fig. 6a and Table 1) resembles

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L. Favaro et al. / Journal of Solid State Chemistry 183 (2010) 901–908
the simulations, may give rise to higher energy bands. Notice
II
III
IV VI
IV V VI
800
that the presence of these modes may be allowed by the
microgranular nature of our samples.
Finally, a small quantity of OH from the initial gibbsite is still
2
1
1
0
.0
.5
.0
.5
1
1
173 K
273 K
present as attested by the presence of small bands assigned to
À1
hydroxyl deformation around 1070 cm
.
0
2
.0
.0
I
II
III
8
6
4
2
00
00
00
00
0
1
1
0
0
.5
.0
.5
.0
2
0
30
40
50
60
70
80
2
θ
(deg.)
Fig. 7. XRD pattern of alumina formed from gibbsite calcined for 24 h at 1573 K
(
expected
a
phase) (lower part) and pattern from 42-1468 JCPDS file (upper part).
0
200
400
600
1000
1200
-1
Wavenumber (cm )
Fig. 6. IR absorbance spectra: (a) alumina formed from gibbsite calcined for 24 h
at 1173 K (expected phase) and 1273 K, (b) calculated spectrum of -alumina:
AlO (B)
AlO6(S) surface modes ?
6
k
k
2
1
.0
.5
sticks are proportional to oscillator strength (maximum intensity normalized at 1)
À1
and line represents a sum of 20 cm
width Gaussians (maximum intensity
normalized at 2). The energy range is separated in six regions.
the theoretical range III as described previously. In particular
À1
the four last peaks at 563, 597, 641 and 657 cm
may
À1
1.0
correspond to the calculated 562, 565, 593 and 620 cm
structures. In contrast, the 480–540 cm range shows three
medium intensity bands at 487, 506 and 536 cm unlike the
À1
À1
0
.5
theoretical predictions presenting one dominating band
À1
(483 cm ) accompanied by smaller structures.
IV. The fourth energy region expands theoretically from 630 to
À1
0.0
2.0
7
6
50 cm . It is clearly dominated by Al–O stretching for AlO ,
in good agreement with other alumina structures, and some
smaller contributions from other modes. More precisely, four
À1
main bands are calculated at 635, 665, 718 and 736 cm . The
first one appears as a shoulder on the last intense peak of the
previous energy region and may correspond, on the experi-
1.5
1.0
0.5
0.0
À1
mental spectra, to the unresolved structure at 657 cm
(see
Table 1). The next three bands may be consistent with the 690,
À1
7
10 and 737 cm
measured ones.
V. The next region includes three modes of Al–O stretching for
À1
AlO
4
, the dominating one being at 778 cm
which may
.
À1
match with the experimental structure at 782 cm
À1
VI. The last region extending above 790 cm
contains again
multiple contributions with maximum oscillator strength
À1
around 835cm . Experimentally, in this region, two bands at
À1
200
400
600
800
1000
1200
8
19 and 838 cm
superimposed on a wide and intense
À1
Wavenumber (cm-1)
background peaking at 880 cm
are observed. This intense
structure.
Two assignments may be suggested. First, a remnant of the
continuum is not predicted theoretically for the
k
Fig. 8. IR absorbance spectra: (a) alumina formed from gibbsite calcined for 24 h at
573 K (expected phase), (b) calculated spectrum of -alumina: sticks proportional
1
a
a
w
structure (see Fig. 4) for which the absorbance range extends
to oscillator strength (maximum intensity normalized at 1), line represents a sum of
À1
À1
up to 900 cm . Alternatively, surface modes, not included in
30 cm width Gaussians (maximum intensity normalized at 2).

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L. Favaro et al. / Journal of Solid State Chemistry 183 (2010) 901–908
907
3
.3. Alpha alumina (corundum)
XRD and IR spectroscopy together with ab initio simulations
allowed studying the sequence: gibbsite - -- -- -Al with
increasing temperature from 773 to 1573 K. No transition from
w
k
a
2 3
O
When heating up at temperature above 1573 K for 24 h, gibbsite
transforms into
pattern of the 1573 K sample (Fig. 7) shows that only
present when compared with 42–1468 JCPDS file. In this structure,
a
-alumina, the stable structure (Fig. 1). The XRD
gibbsite to boehmite together with the subsequent transition
a
-Al is
2
O
3
2 3
boehmite -g--d--y--a-Al O was observed. The evolution
gibbsite - -Al O is resumed in Fig. 9(a and b) presenting the
a
2
3
3
+
tetrahedral Al
ions are no longer present and only AlO
6
XRD patterns and IR spectra along the sequence. Both techniques
show that heating gibbsite, a very well crystallized solid, at 773,
1173 and 1573 K, yields respectively the semi-amorphized
octahedron remain. Accordingly, the measured IR spectra
corresponding to this temperature (Fig. 8a) shows a simple
spectrum as was observed in previous works [23, 3], with four
w
phase, followed by the complex but well crystallized
k phase
À1
main structures visible between 350 and 650 cm . The lowest
and finally the simple and highly crystallized stable -Al O U
a
2
3
À1
energy peaks (under 500 cm ) are assigned to Al–O bending for
At 773 K, infrared spectra shows a wide structure with three
À1
À1
AlO
6
, the bands above 500 cm resulting from Al–O stretching (see
broad unresolved bands extending between 200 and 900 cm in
Table 1, third column). The comparison with equivalent spectrum
simulation (Fig. 8b) shows that the main bands are well predicted
close resemblance with the equivalent spectra of g phase [3]. In
contrast, the XRD measurements allow distinguishing these two
confirming indeed the assignment above. Nevertheless, some
transition alumina. The absence of a reliable crystallographic
structure for the w-alumina prevents ab initio calculations and
À1
discrepancies can be noticed. Firstly, a shoulder at 484 cm
present on the higher energy side of the bending mode at 449 cm
is
À1
only experimental results are available.
on the experimental spectrum. More importantly, a wide and
As shown by XRD analysis, at 1173 K,
initio calculations predict a very complex IR spectrum with a large
number of bands extending between 130 and 930 cm , in good
qualitative agreement with the measured spectra. This complexity
reflects the large number of different crystallographic non equivalent
k-alumina is formed. Ab
À1
intense continuum dominates the 650–800 cm
region where no
À1
structures are predicted by our calculations. This high energy
structure clearly resembles the high energy continuum already
observed on the
k spectrum. In analogy with the suggestion made
for the latter case, we suggest these extra structures to result from
either surface modes, residual water or remnant transition phase
modes, although no transition alumina is detected from the XRD.
atoms (four Al and six O) accompanied by the wide range of
internuclear distances (extending from 1.72 to 2.27 A˚ ) and also the
various coordinance (AlO , AlO (5+1), AlO , OIII, OIV and OV). Notice
6
6
4
Notice that one can also exclude contributions from longitudinal
that this last oxygen coordinance (OV) is specific for k- transition
À1
optical (LO) modes as these are calculated around 900 cm
.
alumina and gives rise to the low energy bands calculated under
À1
2
00 cm . Another consequence of this complexity is the departure
from the simple band energy sequence: Al–O bending for AlO
bending for AlO , Al–O stretching for AlO and Al–O stretching for
AlO , as was evidenced for other forms of alumina [3, 24] and
4
, Al–O
4
. Conclusions
6
6
4
references therein. Nevertheless, from the ab initio calculations, six
energy regions have been distinguished according to their main
The aim of this work was to study the alumina phase resulting
from thermal treatment of gibbsite. The combined use of both
10
20
30
40
50
60
70
80
200
400
600
800
1000
1
1200
1573 K
573 K
1
1
273 K
1273 K
1173 K
173 K
773 K
773 K
1
0
1
gibbsite
70 80
gibbsite
1000 1200
0
200
10
20
30
40
50
60
400
600
800
-
1
2
θ (deg.)
Wavenumber (cm )
Fig. 9. XRD patterns (a) and IR absorbance spectra (b) for gibbsite and for alumina formed from gibbsite calcined for 24 h at 773, 1173, 1273 and 1573 K. All intensity
maxima are normalized at 1.

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L. Favaro et al. / Journal of Solid State Chemistry 183 (2010) 901–908
features and a tentative assignment was suggested, based on
similarities between theoretical and experimental spectra.
[3] A. Boumaza, L. Favaro, J. L e´ dion, G. Sattonnay, J.B. Brubach, P. Berthet, A.M.
Huntz, P. Roy, R. T e´ tot, J. Solid State Chem. 182 (2009) 1171.
[
[
4] B. Whittington, D. Ilievski, Chem. Eng. J. 98 (2004) 89.
5] J.T. Kloprogge, H.D. Ruan, R.L. Frost, J. Mater. Sci. 37 (2002) 1121.
When heating gibbsite at 1573 K, as expected, the stable
a
-alumina forms. For this phase, a good agreement is observed
between experimental and simulated IR spectra.
For both and phases, extra modes unpredicted by ab initio
calculations are observed experimentally. They can be explained
by either remnant transition alumina or surface modes. A
[6] R.L. Frost, J.T. Kloprogge, S.C. Russel, J.L. Szetu, Appl. Spectrom. 53 (1999)
23.
4
[
[
7] K. Wefers, C. Misra, Alcoa Technical Paper No. 19 (1987) (revised).
8] J. Rouquerol, F. Rouquerol, M. Ganteaume, J. Catal. 36 (1975) 99.
k
a
[9] R. Dovesi, V.R. Saunders, C. Roetti, R. Orlando, C.M. Zicovich-Wilson,
F. Pascale, B. Civalleri, K. Doll, N.M. Harrison, I.J. Bush, Ph. D’arco, M. Llunell,
Crystal 06 User’s Manual, 2006, University of Torino, Torino.
10] S.H. Vosko, L. Wilk, M. Nusair, Can. J. Phys. 581 (1980) 200.
11] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
w
specific study on these surface modes is in progress [25].
The precise characterization made in this paper and in the
previous one [3] may constitute a basis for investigations on thin
layers of alumina form under various experimental conditions on
alumina-forming materials [25].
[
[
[12] M. Catti, G. Valerio, R. Dovesi, M. Causa’, Phys. Rev. B 49 (1994) 14179.
[
[
13] B. Civalleri, P. Ugliengo, J. Phys. Chem. 104 (2000) 9419.
14] F. Pascale, C.M. Zicovich-Wilson, F. Lopez Gejo, B. Civalleri, R. Orlando,
R.J. Dovesi, Comput. Chem. 25 (2004) 888.
[15] B. Montanari, B. Civalleri, C.M. Zicovich-Wilson, R. Dovesi, Int. J. Quantum
Chem. 106 (2006) 1703.
[
[
[
[
16] H. Saalfeld, M. Wedde, Zeitschrift f u¨ r Kristallographie 139 (1974) 129.
17] T. Kogure, J. Am. Ceram. Soc. 82 (1999) 716.
18] E. Balan, M. Lazzeri, G. Morin, F. Mauri, Am. Mineral. 91 (2006) 115.
19] H.C. Stumpf, A.S. Russel, J.W. Newsome, C.M. Tucker, Ind. Eng. Chem. 42
(1950) 1398.
Acknowledgments
We wish to thank Celine Byl and Michaele Herbst (ICMMO
(LEMHE)) for technical support and Ross Gash and Laurent
Manceron for a critical reading of the manuscript. We also thank
the Division des Ressources Informatiques (DRI) of SOLEIL and the
Direction Informatique (DI), Orsay, for providing computational
resources.
[20] G.W. Brindley, J.O. Choe, Am. Mineral. 46 (1961) 771.
[
[
21] H.N. Saalfeld, Jb. Miner. Abh. 95 (1960) 1–87.
22] B. Ollivier, R. Retoux, P. Lacorre, D. Massiot, G. F e´ rey, J. Mater. Chem. 7 (1997)
1
049.
[23] P. Tarte, Spectrochem. Acta 23A (1967) 2127.
[
[
24] P. Colomban, J. Mater. Sci. 24 (1989) 3002.
25] L. Favaro, P. Roy, E. Amzallag, R. T e´ tot, in preparation.
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